During exploration and reservoir assessment and development in the oil and gas industry, hydrocarbons, such as oil and gas, as well as geological structures that tend to bear hydrocarbon, may be detected based on their properties (e.g., mechanical and electromagnetic (EM) properties) that are different from those of the background geological formations.
Electromagnetic (EM) measurements are commonly used in oil and gas exploration. Among the EM properties, the resistivity (ρ), which is an inverse of the electrical conductivity (σ), is particularly useful. This in because hydrocarbon-bearing bodies, such as oil-bearing reservoirs, formations containing methane hydrate, and gas injection zones, have higher resistivities compared with their background geological formations. For example, hydrocarbon-bearing reservoirs typically have resistivities one to two orders of magnitude higher than those of the surrounding shale and water-bearing zones. Therefore, resistivity mapping or imaging may be useful in locations the zones of interest in contrast to the background resistivity. This method has been used successfully in both land and seafloor exploration.
Resistivity mapping may be achieved by generating an EM signal above the formations of interest and receiving the resulting EM field at selected locations. The received data is affected by a number of parameters, for example, the distance between the EM signal source and the receivers, EM field frequency, polarity of the EM waves, depth and thickness of the reservoir, resistivity of the hydrocarbon bearing zones, and the surrounding geological formations. In marine applications, the received signal may depend on the resistivity of the seawater, which may be a dynamic variable that depends on the water temperature, salt content, etc.
The EM signal may be from natural sources or from artificial sources. Among the EM methods, magneto-telluric (MT) methods rely on the naturally-occurring EM fields in the stratosphere surrounding the earth. Because carbonates, volcanics, and salt all have large electrical resistivity as compared with typical sedimentary rocks, MT measurements may produce high-contrast images of such geostructures. MT measurements are particularly useful in examining large-scale basin features and for characterizing reservoirs below basalt (volcanics) layers beneath a sea bed.
Controlled source electromagnetic (“CSEM”) methods use EM transmitters, called sources, as energy sources, and the receivers measure the responses of the geological structures to the transmitted signals. The transmitter may be a direct current (DC) source, which injects a DC current into the geological formations. DC currents are typically injected into the formations using contacting electrodes. Recent EM measurement methods use EM sources that produce time-varying electrical and/or magnetic (EM) fields. The EM fields may be a pulse generated by turning on and off an EM transmitter, and in this case, the receivers effectively measure a pulse response of the geological structures. EM measurements may sue a transmitter that transmits a fixed frequency or a range of frequencies. The higher frequency EM sources permits resolution of finer structures, whereas the lower frequency EM sources allows one to probe deeper into the formations.
In marine explorations, low-frequency EM methods are typically used. The low-frequency EM waves may induce a current, i.e., the Faraday (eddy) current, to flow in the earth formation and in the sea water. The current density depends on the resistivity of the earth formation and the sea water. A voltage drop across two locations produced by the current may be measured and used to infer the resistivity distribution in the formation. Alternatively, one may measure the secondary magnetic fields produced by the induced current.
As discussed, CSEM uses an artificial EM source to generate controlled EM fields that penetrate the ocean and the subsea formations. As illustrated in FIG. 1, in a conventional CSEM method, an electrical dipole transmitter 11 is towed by a ship 10, typically at a short distance above the seabed 12. In other cases, the transmitter 11 may be towed near the surface or at other depths. The transmitter 11 induces EM fields in the sea water 14, geological layers 15 and 16 and the oil-bearing layer 17.
To detect the EM signals, a number of receivers 13 are deployed on the seabed 12. The EM signals measured by the seafloor receivers 13 may then be used to solve the resistivity distributions in the geological structures, including layers 15, 16, and 17. When the transmitter 11 is not used, the receivers 13 may be used to detect EM signals induced by the naturally-occurring MT fields.
A traditional receiver used in such surveys measures a voltage drop across a short distance. As illustrated in FIG. 2, such receivers typically uses a voltmeter B to measure the voltage drop at a selected distance L, i.e., at locations A and C. The voltage drop (ΔV) across A and C, as measured by the voltmeter B, is then used to estimate the electric field E. As a result, the electric field (E) can be simplified as a voltage drop between locations A and C divided by the distance between A and C.
  E  =            (                        V          C                -                  V          A                    )        L  
The sensitivity of a receiver depends on the strength of the signals detected. Because seawater is very conductive, the voltage drop across the measurement points (i.e., A and C) will be very small. For the same electric field E, the detected voltage ΔV would be larger if the distance L between the locations A and C is larger because ΔV=L E. However, it is impractical to increase L beyond a certain limit for the purpose of increasing the sensitivity of the measurements. This is because it will be more difficult to transport and deploy large-sized receivers, and the reliability of the receivers also suffers.
Due to the technical difficulties in measuring the electric fields by voltage drops, it may be more advantageous to measure an electric field E by measuring electric current densities J and the electric conductivity σ of the sea water. Then, the electric field E may then be derived using the Ohm's law,E=J/σ,   (1)where J is the current density, and σ is the electric conductivity. This principle has been applied to measuring electric fields using opposing conductive plates in a cubic or rectangular receiver frame, as taught in French Patent 8419577, issued to Jean Mosnier, and in WO 2006/026361 by Steven Constable. This French Patent and the WO 2006/026361 are incorporated by reference in their entireties. One example of such a receiver is illustrated in FIG. 3.
As shown in FIG. 3, a receiver device 30 includes electrodes 31 and 32 disposed on opposite sides of the cubic frame. The electrodes are connected via a circuitry 33 having an impedance Z, which may be tuned such that the impedance of the receiver is identical to that of the seawater 34. If the impedance between the electrodes 31 and 32 is matched to that of the seawater, then the presence of the receiver in the seawater will not perturb the electric field of the measurement site. Therefore, the current I that passes through electrodes 31 and 32 will be the same current that would have passed through the space occupied by the receiver 30, if the receiver 30 were not present.
Although it is desirable to tune the receiver impedance Z to that of the surrounding seawater, this often is impractical because the seawater resistance may not be known beforehand. Furthermore, the resistance (or conductivity) of seawater can vary with time, temperature, salt concentration, etc. Although Mosnier disclosed a way to overcome this problem by using a mechanical device to open and close, at regular intervals, the communication between one of the electrodes and the outside environment, this approach is not practical because it requires a substantial amount of energy.
In addition to the above described problem, the sensitivities of the receivers of Mosnier may be low due to various factors, such as noises generated by the receiver itself or from the environment. Therefore, while the prior art receivers have been useful in oil and gas exploration, there remains a need for better receivers that are easy to use and can provide robust measurements.